The present invention relates to an optical reproducing apparatus which is arranged to optically utilize the direction of polarization of light reflected from an optical recording medium such as a photo-electromagnetic disk to optically reproduce information recorded on the optical recording medium or to optically record information on the same.
In general, in order to reduce the access time of an optical disk apparatus of this type, it is important to reduce the size and the weight of the entire optical system of an optical pickup. However, in conventional arrangements of the type in which a track signal, a focus signal and a photo-electromagnetic signal are detected by individual optical systems, a large number of optical parts, and hence a complicated structure, are required. As a result, the weight of the optical pickup increases and the access time inevitably becomes long. In such a situation, an attempt has been made at reducing the size and the weight of the optical pickup by utilizing a high-density grating as a part of the optical pickup. In this system, however, because of the high-density grating, the diffraction angle of diffracted light tends to deviate to a great extent due to fluctuations in the wavelength of the laser light fed from a semiconductor laser. Moreover, two photosensor elements, one for detecting a focus signal and the other for detecting a track signal, are spaced apart from each other by a great distance, thus leading to difficulty in assembly or adjustment.
In light of the above problems, in U.S. patent application Ser. No. 294,466 filed on Jan. 6, 1989, the present applicant proposes an optical pickup which employs a dual grating as the light separating means to realize a light-signal detecting function in the form of a compound integral arrangement. In this optical pickup, it is possible to greatly reduce changes in a diffraction angle due to wavelength fluctuations in a light source, since two gratings of the dual grating serve to cancel such wavelength fluctuations with each other by means of its dual structure. In this optical pickup, the light reflected from an optical disk passes through an objective lens and a polarizing beam splitter, being incident on a focusing lens, and then on the dual grating. This dual grating is of a light-transmitting type in which a first grating and a second grating are formed on opposite surfaces of one substrate integrally therewith, and the first grating arranged as a high-density grating is located on a side nearer to the focusing lens, while the second grating arranged as a modulation pitch grating is located on the outgoing side, from which the light exits. The dual grating is disposed in such a manner as to be inclined by a predetermined angle with respect to the optical axis of the light reflected from the optical disk.
In such a construction and arrangement, the light focused on the dual grating by the focusing lens is separated into a diffracted light and a transmitted light. A focus signal is detected by an astigmatism method utilizing a four-split type photosensor element for receiving the transmitted light, while a track signal is detected by a push-pull method utilizing a two-split type photosensor element for receiving the diffracted light. In addition, a particular photo-electromagnetic signal on the optical disk is picked up by obtaining the difference between the detection outputs of the respective photosensor elements.
In accordance with the above-mentioned system employing the dual grating, even if wavelength fluctuations occur in the laser light emitted from the semiconductor laser, the diffracted light exiting from the dual grating suffers little offset and is therefore insusceptible to the influence of the wavelength fluctuations because the first grating and the second grating serve to cancel the wavelength fluctuations with each other. In addition, it is possible to provide an optical pickup consisting of a small number of parts and having a relatively small size.
The above-mentioned system employing the dual grating, however, has the following problems.
The first problem is that, even if the pitch of diffraction pattern on the first grating is made equal to the pitch of a diffraction pattern on the second grating so that fluctuations in the angle of diffraction will become small, the fluctuations still exist in some degree because it will occur while the diffracted light from the first grating is traveling in the dual grating to the second grating. Accordingly, the influence of the wavelength fluctuations can not be effectively eliminated.
The second problem is that a great limitation is imposed on any reduction in the overall size of the optical system of the optical pickup. If the optical pickup of this system is to be further miniaturized, it is necessary to employ a focusing lens having a small focal length f, but, since the interval between the photosensor element for detecting a track signal and the photosensor element for detecting a focus signal is definite, the angle of separation between the transmitted light and the diffracted light in the dual grating inevitably becomes large and the resistance of the dual grating to wavelength fluctuations deteriorates. As a result, the performance of the entire optical pickup is impaired. It follows, therefore, that a reduction in the overall size of the optical system of the optical pickup is limited to a great extent.